Population Units

From EU3 Wiki

The 'Units' value displayed in each province is a modifer which is based on the province's population.

For colonies, Units = 0.1 * (colony_level)

For cities, Units = 0.995 + population/200000 to a maximum value of 2.0 which occurs when the population is 201,000 or higher.

This modifier affects manpower, production (and gold) income, toll income, and the value of trade the province contributes to a center of trade.